Posted too soon obviously
So you'd be willing to risk your relationship, his marriage, your sons wellbeing and trust in you, his job and possibly professional registration and therefore entire career for a mere shag?
"No one will find out" famous last words of many a cheat.
If you have a genuinely open relationship then you can easily find sexual partners for a little fun on the side without risking most of what I listed earlier.
no one’s going to fall in love and leave anyone?
You can't possibly guarantee that.
I'm involved in an ethical non monogamy community/circle and this would still be very much frowned upon and criticised because his wife has not consented to or agreed to any of this, doesn't even know. She has a right to whether you like it or not.
Even from a selfish perspective, if the wife does find out and tips up angry/upset at your door potentially while your young kids are there what would be your defence to that?
And he has a child too so you're potentially risking the negative impacts on the child.
Very selfish to go ahead I think.